1. Field of the Invention
The present invention relates to a process for preparing a lithographic printing plate, and more particularly, to an improvement in the characteristics of a lithographic printing plate comprising a support having thereon at least two kinds of materials which are mutually diffusible upon exposure to light.
2. Description of the Prior Art
As described in U.S. Pat. Nos. 3,637,377-81, 3,637,383, 3,663,224, 3,650,743 and 3,707,372 and Japanese Patent Application (OPI) No. 10018/74 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), it is known that, when a light-sensitive element comprising a support having thereon a layer of a light-sensitive chalcogen or a certain kind of metal halide and a layer of a metal adjacent thereto is exposed to light, a mutual reaction occurs between both layers. If such a photoreaction occurs, the corresponding portions mutually diffuse into each other (form a mutually diffused state, the so-called photodoping phenomenon), and, as a result, various properties such as electric properties, e.g., electric resistivity, photoconductivity, photoelectromotive force, etc., light transmittance, light reflectance, solubility in an alkaline solution, hydrophilic or oleophilic properties and the like become different at the diffused portions from those of the original material. Property changes in this kind of a lightsensitive material due to light are extremely useful from a photographic technique standpoint, and are being utilized in a variety of fields where applicable. One of the major fields where such can be utilized is in producing lithographic printing plates.
For example, U.S. Pat. Nos. 3,650,743 and 3,707,372 disclose that if a radiation sensitive element comprising a support having thereon a first layer comprising a chalcogenide or a metal halide compound with a second layer comprising a metal laminated thereon is imagewise exposed to light, image areas with hydrophilic or oleophilic properties different from those at the non-exposed areas are formed, and, hence, a lithographic printing plate can be prepared merely by wetting the printing surface prior to inking, without any other processing being required. Therefore, the lithographic printing plate described in U.S. Pat. Nos. 3,650,743 and 3,707,372 does not require any processing following exposure, such as development, gum coating, lacquering, etc., which have been essential in the conventional production of lithographic printing plates using diazonium compounds, etc., so that it is very advantageous from the standpoints of time, cost and prevention of pollution. However, this lithographic printing plate has a disadvantage that the quality of the prints is far inferior to lithographic printing plates using diazonium compounds since the difference between the hydrophilic or oleophilic property exhibited by the mutual diffusion products described above and the oleophilic or hydrophilic property at the non-exposed areas is small in this lithographic printing plate.
Lithographic printing plates where this disadvantage has been minimized to some extent are described in Japanese Patent Applications (OPI) Nos. 125803/75 and 15703/77. However, the disadvantage described above has not been minimized sufficiently even in these lithographic printing plates.